(CNN) -- Democratic presidential hopeful Barack Obama, moving into front-runner status following a week of eight straight wins, is facing a new rival, exchanging fire with John McCain, the presumptive GOP nominee.

Sen. Barack Obama speaks to a rally at the University of Wisconsin-Madison Tuesday.

In what could be a preview of the general election, the two exchanged jabs over Iraq and the economy, sure to be key issues in November.

McCain criticized the Democrats for speaking in "platitudes" about Iraq.

"The Democrats wanted to leave and set a date for withdrawal and said we could never succeed militarily. Look at the record ... not the rhetoric, not the platitudes, but the principles and the philosophy," he said.

McCain later took aim directly at Obama for lacking specifics.

"I respect him and the campaign he has run," McCain said.

"But there is going to be time when we have to get into specifics, and I have heard not every speech he has given obviously, but they are singularly lacking in specifics, and that's when as the campaign moves forward, we will be portraying very stark differences."

Those comments are among McCain's most pointed attack at Obama to date, a clear sign the Republican nominee apparent is increasingly viewing the Illinois senator as the Democratic front-runner.

"If you want the same as we've had in the last seven years, then I think John McCain's going to be a great choice," Obama said. "But if you think we need something new ... then I hope that you will stand with me and vote for me."

Meanwhile, Obama faced criticism from Clinton, who stepped up her campaigning Wednesday with fresh attacks of her own, directly challenging Obama's ability to deliver on his rhetoric.

"I am in the solutions business. My opponent is in the promises business," the senator said in McAllen, Texas. "It's not the questions. It's the answers. And the answers get right to the heart of who is ready on Day One to be the president and commander-in-chief of the United States."

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Texas holds its primaries on March 4, along with Ohio, Vermont and Rhode Island. Clinton has events scheduled in McAllen, Robstown and San Antonio, Texas.

In addition to Texas with its large Latino population -- a group she has done well with this primary season -- Clinton is also banking on winning Ohio and its blue-collar voters, another group that has backed her in previous primaries and caucuses. Clinton was scheduled to campaign in Ohio on Thursday. Watch the Clinton's camp views of race ahead »

Obama was scheduled to stump on Wednesday in Wisconsin, which will hold its primaries next Tuesday. Voters in Hawaii, Obama's native state, will also head to the polls on that day.

Recent polls show a tight race between the two Democratic front-runners in Wisconsin, but a Strategic Vision polls conducted February 8-10 has Obama leading Clinton 45-41, a lead outside of the poll's margin of error of 3 percentage points.

The Wisconsin contest picked up in intensity Wednesday when the Clinton camp launched a television ad criticizing Obama for not agreeing to debate in the state.

"Both Democratic candidates have been invited to a televised Wisconsin debate," an announcer says in the Clinton ad. "Hillary Clinton has said yes. Barack Obama hasn't. Maybe he'd prefer to give speeches than have to answer questions."

Responding to the ad, Obama campaign manager David Axelrod said, "We've debated 18 times, we're going to debate two more, but we've got other business to do here in terms of meeting voters face to face."

While campaigning in Wisconsin, Obama took his own shots at Clinton, criticizing her for supporting the North America Free Trade Agreement while former President Clinton was president.

"You know, in the years after her husband signed NAFTA, Sen. Clinton would go around talking about how great it was and how many benefits it would bring," Obama said in Janesville, Wisconsin. "Now that she's running for president, she says we need a time-out on trade. No one knows when this time-out will end -- maybe after the election."

Also on Wednesday, in what could be seen as recognizing a second-place status, the Clinton camp sent out a memo pointing out other eventual nominees who suffered big losses before clinching the nomination.

"As history shows, the Democratic nomination goes to the candidate who wins the most delegates -- not the candidate who wins the most states," the memo said.

The memo calls attention to the upcoming contests in Ohio, Texas and Pennsylvania.

Rich Galen, a Republican strategist, said the note is "a memo sent out by a campaign that knows it's in trouble."

"It's not clear to me why they have set the bar so high for themselves in Ohio and Texas on March 4, but they've done it, and now they've got to pay it off," he said on CNN's "The Situation Room."E-mail to a friend